Degenerative Myelopathy (DM)
Degenerative Myelopathy is an inherited neurologic disorder caused by a mutation of the SOD1 gene known to be carried by American Staffordshire terriers. This mutation is found in many breeds of dog, though it is not clear for American Staffordshire terriers whether all dogs carrying two copies of the mutation will develop the disease. The variable presentation between breeds suggests that there are environmental or other genetic factors responsible for modifying disease expression. The average age of onset for dogs with degenerative myelopathy is approximately nine years of age. The disease affects the White Matter tissue of the spinal cord and is considered the canine equivalent to amyotrophic lateral sclerosis (Lou Gehrig’s disease) found in humans. Affected dogs usually present in adulthood with gradual muscle Atrophy and loss of coordination typically beginning in the hind limbs due to degeneration of the nerves. The condition is not typically painful for the dog but will progress until the dog is no longer able to walk.
The gait of dogs affected with degenerative myelopathy can be difficult to distinguish from the gait of dogs with hip dysplasia, arthritis of other joints of the hind limbs, or intervertebral disc disease. Late in the progression of disease, dogs may lose faecal and urinary continence and the forelimbs may be affected. Affected dogs may fully lose the ability to walk 6 months to 2 years after the onset of symptoms. Affected medium to large breed dogs, such as the American Staffordshire Terrier, can be difficult to manage and owners often elect euthanasia when their dog can no longer support weight in the hind limbs. The mutation of the SOD1 gene associated with degenerative myelopathy has been identified in American Staffordshire Terriers. The overall frequency of this disease in the breed and approximate age of disease onset are unreported for American Staffordshire Terriers. However, in one study of 24 American Staffordshire Terriers tested, 8.3% were carriers of the mutation and 29.2% were at-risk.
Genetic testing of the SOD1 gene in American Staffordshire Terriers will reliably determine whether a dog is a genetic Carrier of degenerative myelopathy. Degenerative Myelopathy is inherited in an Autosomal Recessive manner in dogs meaning that they must receive two copies of the mutated gene (one from each parent) to develop the disease. In general, carrier dogs do not have features of the disease but when bred with another carrier of the same Mutation, there is a risk of having affected pups. Each pup that is born to this pairing has a 25% chance of inheriting the disease and a 50% chance of inheriting one copy and being a carrier of the SOD1 gene mutation. Reliable genetic testing is important for determining breeding practices. Because symptoms may not appear until adulthood and some at-risk/affected dogs do not develop the disease, genetic testing should be performed before breeding. Until the exact modifying environmental or genetic factor is determined, genetic testing remains the only reliable way to detect neurological disease associated with this mutation prior to death. In order to eliminate this mutation from breeding lines and to avoid the potential of producing affected pups, breeding of known carriers to each other is not recommended. American Staffordshire Terriers that are not carriers of the mutation have no increased risk of having affected pups.